In recent years, many techniques that involve use of micro-magnetic particles as carriers and various reactions in solutions have been developed. Various techniques using magnetic particles, for example, an immunoassay (see, for example, J. Immnunol. Methods, 218:1-2, 1-8, Sep. 1 (1998)), a method of extracting and analyzing nucleic acids (see, for example, Biotechniques, 13:1, 124-31, July (1992)), a method of analyzing proteins or ligands, chemical reactions such as combinatorial chemistry, have been developed and extensively used.
For example, among various immunoassays that are extensively used as methods of detecting various diseases in early stages and methods of detecting trace amounts of substances, techniques that involve use of magnetic particles having carried thereon antigens or antibodies are highly evaluated since the techniques give high sensitivities and allow simple operation of B/F separation and so on. “B/F separation” means a step of separating an antigen-antibody reaction product from an unreacted substance by discarding the reaction mixture containing the unreacted substance from a reaction vessel such as a reaction tube or wells of a microtiter plate and repeating a washing operation that includes supplying and discarding a washing solution. When magnetic particles having carried thereon an antigen or an antibody are used, the magnetic particles and a sample are mixed to perform an antigen-antibody reaction, and then B/F separation including a step of collecting the magnetic particles that contain the generated immune complex and a step of separating and washing unreacted antigen or antibody can be performed easily and quickly using magnetic force.
Specifically, a method that involves contacting the magnetic material with a reaction vessel or the like that is in a state of stand still, such as a reaction tube or wells of a microtiter plate, and collecting magnetic particles (see, for example, JP 3-144367 A); a method which involves collecting magnetic particles by contacting a magnetic material with a tip, a stainless steel pipe, or a flexible tube used as a flow line for sucking/discharging liquid from/into a vessel (see, for example, JP 3115501 B); and many other reports have been made.
However, for these methods, it is an important problem to improve the recovery rate (collection efficiency) of the magnetic particles. Although the micro-magnetic particles are useful for performing various reactions at a high efficiency, they tend to float in the solution, which tends to lead to a reduced recovery rate. When the recovery rate decreases, the magnetic particles may flow outward in the processes of washing, separation and so on to cause an error. This may reduce the reliability of measured values. However, when the washing is insufficient with a view to preventing the magnetic particles from flowing outward, complete separation cannot be attained. In addition, samples after separation, for example, pretreated samples are used, contamination may occur. In particular, in the case of the immunoassays, the B/F separation is performed a plurality of times and if the outward flow of the magnetic particles or errors occur, the measured values are greatly influenced.
To solve those problems, for example, a method that involves settling the magnetic particles over a long time until all the magnetic particles are collected without fail has been tried. Surely, this method allows one to collect the magnetic particles in a relatively short time when a reaction vessel in which the reaction mixture stands still, such as a reaction tube or a microtiter plate, is used. However, when the reaction mixture flows as in the case of a flow line that sucks liquid from the vessel and discharges the liquid, it is difficult to improve the recovery rate. In addition, the above-mentioned method requires a long time for measurement, so the method is unsuitable for clinical tests or the like for which rapid measurements are required.
On the other hand, a method of enhancing the magnetic force using electromagnets or permanent magnets with enlarged sizes that are employed as a magnetic material for collecting the magnetic particles has been tried. However, this method is not practically acceptable since it has a problem, for example, that a large apparatus is necessary.
As described above, it is hardly to say that a method of collecting magnetic particles with ease and in a high efficiency, which method exhibits a high reproducibility even in the immunoassays that involve a plurality of B/F separation operations, has been already established. Accordingly, development of a magnetic material that is small and can collect magnetic particles in a high efficiency is demanded. In particular, in the field of Point of Care Testing (POCT) and the like that are attached importance in recent years in clinical tests, demanded is an apparatus that is small and easy to handle and with which medical physicians and nurses can perform tests quickly. Accordingly, down-sizing of the apparatus is essential. In addition, it is required that the apparatus used in the field of POCT is adapted to use whole blood immediately after taking the blood sample as it is to achieve tests quickly. However, when the whole blood containing a lot of contaminant proteins is used as a sample, the magnetic particles tend to aggregate, which leads to a decrease in the recovery rate of the magnetic particles. Accordingly, it is keenly demanded to perform B/F separation at a high recovery rate using strong magnetic material.